In the prior art, displays generally comprise cathode ray tube (CRT) displays and liquid crystal displays (LCDs). With the development of science and technology, organic light-emitting diode (OLED) displays gradually come into the people's daily lives.
Currently, CRT displays have been gradually replaced by the liquid crystal displays and OLED displays with a high cost performance. In an imaging process, each liquid crystal pixel in a liquid crystal display is driven by a thin film transistor (TFT) integrated in a thin film transistor substrate and cooperates with a peripheral drive circuit to achieve an image display. In an Active Matrix Organic Light Emission Display (AMOLED), an OLED pixel in an OLED panel is driven by a corresponding TFT in a TFT substrate and cooperates with a peripheral drive circuit to achieve an image display. In the above display, as a switch for controlling the light emitting, the TFT is the key to achieve a liquid crystal display and an OLED display with large size, which is directly related to the development direction of high-performance flat panel displays.
The TFT is usually made of a semiconductor material, and can be classified into the following types according to its semiconductor material for preparing the active layer: inorganic TFT, organic TFT, and inorganic/organic composite type TFT. Currently, the mostly used type is inorganic TFT using silicon based material, compounds, metal oxides and other materials. In the existing flat panel display manufacturing technology, the TFT which has been used in industrialization mainly employs materials such as amorphous silicon, polycrystalline silicon, mono-crystalline silicon and other silicon-based materials. Array substrate used in manufacturing flat panel display mostly uses the amorphous silicon TFT. However, with the increasing size of flat panel displays and the increasing frequency of the driving circuit, the mobility of the TFT is required to be sufficiently high. This is because if the TFT can be made smaller, the resolution of the flat panel display can be greater, and the display effect can be better due to the higher mobility. However, the existing amorphous silicon TFT has a limited carrier mobility, which leads to a longer charging time. Although the existing amorphous silicon TFT can be satisfying in small-size flat panel display, but it is difficult to meet higher display quality and driving requirements in the large-size flat panel display. Therefore, researchers have pay attention to the metal oxide materials which can endow the TFT with higher carrier mobility.
In the case of preparing an active layer of a TFT using the metal oxide material, the supply of the carrier is increased due to lack of oxygen in the metal oxide semiconductor. Therefore, the metal oxide semiconductor has a high carrier density, such that the TFT made of metal oxide has advantages of high carrier mobility. Thus, the TFT can be made smaller, which enables a higher resolution and a better display effect of the flat panel display; simultaneously, the TFT made of the metal oxide have advantages such as few uneven characteristic phenomenon, decreasing cost of materials and processes, lower process temperature, coating process being available, higher transmittance, and higher band gap.
In the prior art, the structure of a metal oxide TFT mainly comprises three types of etch stop type, back channel etch type and coplanar type. These three types of metal oxide TFT generally includes a gate layer, a gate insulating layer, an active layer, a source/drain electrode layer, a passivation layer, and a transparent electrode layer. Currently, the commonly used is the etch stop type metal oxide TFT, and the mostly used metal oxide for preparing the active layer in the etch stop type metal oxide TFT is IGZO (In—Ga—Zn—O). As the etch stop type metal oxide TFT is also formed with an etch stop layer above the active layer so as to prevent the metal oxide IGZO of the active layer from being damaged during the formation of source and drain electrodes, thereby improving the performance and stability of the metal oxide TFT. However, since the metal oxides are poor in stability in the air and is sensitive to oxygen and water vapor, and the oxygen and water vapor can still pass through the etch stop layer so as to deteriorate the performance of the metal oxide in the actual use, the performance of the metal oxide TFT with the active layer formed of metal oxide is decreased, resulting in decreased performance of the metal oxide TFT array substrate and affecting the display quality of a flat panel display.
Currently, the etch stop type metal oxide TFT is usually manufactured by employing a 6-mask patterning process, i.e. using the etch stop layer, in order to prevent the active layer made of the metal oxide from being damaged when the source/drain electrodes are etched. In the metal oxide TFT in the prior art, the gate insulating layer below the active layer is a single layer and prepared by using a SiNx material, and the etch stop layer over the active layer is also a single layer and prepared by using a SiOx material. The difference in etching rate of the SiNx material and the SiOx material under the same condition is relatively high, for example, assuming that the etching rate of SiOx is 1, the etching rate for the SiNx may reach 10 or more. Therefore, in the actual production process, the gate insulating layer not being covered by the active layer pattern is exposed after etching process on the SiOx layer in the etch stop layer is completed. In addition, in the normal preparation process, the uniformity among the layers will be considered, and the etch stop layer is then etched for a more period of time, i.e. the time for over etching. For example, if the etch process on the etch stop layer with a 500 Å thickness needs 50 s to be completed, generally, it will spend 30% of the time (15 s) for over etching. In the over etching process, the gate insulating layer will be inevitably etched to a relatively high extent, so that the over etching has a relatively high impact on the gate insulating layer and the reject ratio of the metal oxide TFT will be increased.
In summary, how to improve the stability of the metal oxide to obtain a metal oxide TFT with high stability, and to improve the stability of the metal oxide TFT array substrate and the display quality of a flat panel display are the current problems to be solved.